Circuits often require a precise voltage level. As such, reference voltage circuits have been developed to generate a precise voltage level that, ideally, does not drift or largely vary with, e.g., temperature changes that the reference voltage circuit may be subjected to. One type of reference voltage circuit, referred to as a bandgap voltage reference circuit, generates a proportional to absolute temperature (PTAT) voltage and a proportional to complementary temperature (CTAT) voltage. The PTAT voltage is derived from a pair of voltages that are generated from different current densities through a P-N junction. The accuracy of the reference voltage that is output by the circuit is most sensitive to the PTAT voltage, since the PTAT voltage is usually multiplied by a certain factor (˜10) to achieve temperature-compensation at the reference voltage output. In turn, the PTAT voltage is proportional to the logarithm of the ratio of the two different current densities.
A problem with integrating a bandgap reference voltage circuit onto a semiconductor chip manufactured with a logic manufacturing process is that the ratio of the two current densities is typically limited to one order of magnitude (e.g., no higher than 50). The relatively small current density ratio results in a bandgap reference circuit that is more sensitive to circuit non-idealities, like amplifier or device mismatch, and may therefore not be suitably accurate or stable for its particular application. Moreover, the ratio of current densities in prior art solutions depends on the matching of devices, like MOS transistors or resistors, which limits the achievable accuracy. In modern technologies it is further difficult to integrate analog structures like resistors, current sources or amplifiers, with sufficient performance (ideality). The requirements for a technology would be relaxed, if a reference circuit does not need such components, but can operate by similar means like the digital core circuitry.